The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Semiconductor lasers are used in a variety of applications, such as high-bit-rate optical fiber communications. To provide optical fiber communications, lasers are optically coupled to fibers to enable modulated light output from the laser to be transmitted into the fiber. Various modules, assemblies or packages are used to hold and align the laser, other optical components (e.g., collimation and coupling lenses, isolators, and the like), and optical fiber such that the laser is optically coupled to the fiber. The process of aligning an optical fiber to a laser and fixing it in place is sometimes referred to as fiber pigtailing. The laser and optical fiber may be aligned and coupled, for example, using a welding machine. Active alignment techniques may be used wherein the power of the light coupled into the optical fiber is measured as the laser and/or fiber is moved to search for the alignment position (e.g., a position of maximum power).
Standard laser package types include butterfly laser packages and coaxial or TO (transistor outline) can laser packages. In a TO can laser package, for example, the laser (e.g., a laser diode) and the light-receiving end of the optical fiber may be mounted together within a substantially cylindrical housing. The laser may be mounted on a laser submount on a TO can post of a TO can header. The fiber end may be disposed in a rigid cylindrical ferrule, which may be welded to the TO can housing after the laser and the optical fiber are aligned.
In this and other types of laser packages, one problem that often arises when a laser is coupled to an optical fiber is back reflection from the end face of the fiber back into the laser cavity. One way to reduce back reflection is to use an angle-polished fiber, which has its end surface polished to a fiber end angle (e.g., 8°) slightly off of the plane normal to the axis of the fiber core. Light from the laser that reflects off of the fiber end, instead of being coupled into the fiber, is reflected at an angle with respect to the axis of the fiber and is thus not reflected back into the laser cavity. One drawback of this approach, however, is that coupling efficiency may be reduced. A primary reason for this reduction in coupling efficiency is that the angled fiber end causes light coupled into the fiber core at the angled end to be bent at a certain refraction angle due to the different indices of refraction of the fiber and surrounding medium (e.g., air). As a result, the light is not coupled into the fiber substantially parallel to the axis of the fiber core, which reduces coupling efficiency.
To improve the coupling efficiency, the light from the laser may be directed at an angle to the angled end of the fiber such that the light coupled into the fiber core is better aligned with the axis of the fiber core. To direct the light at an angle to the angled end of the fiber, the laser may be positioned “off axis” relative to the axis of the fiber core. With such an “off-axis” configuration, aligning a laser relative to the angle-polished fiber may be more difficult. Because the laser is “off-axis” relative to the optical fiber to direct the light at an angle, the proper alignment of the laser is more sensitive to the angular position of the laser. When the laser package is positioned in a welding machine, for example, there may be a rotational shift in the alignment position of the laser, resulting in misalignment. Slight deviations from an alignment position (e.g., a position of maximum power) may result in a significant drop in measured power.
When the laser is “off-axis” relative to the optical fiber to direct the light at an angle to the angled end of the fiber, the use of existing active alignment techniques, such as randomly searching power, may require additional time to find maximum power at different angles. In one system, for example, randomly searching power may require 20 to 25 seconds to find maximum power and position after each rotation and power must be checked at different angles at least 5 times. Such techniques often take considerable amounts of time to find the desired position.